(1) Field of the Invention
The present invention relates to a freshness prolonging device for foodstuff which prolongs a shelf life of foodstuff by storing foodstuff in a high electric field atmosphere.
(2) Description of Related Art
When foodstuff such as fresh foodstuff (e.g., vegetables, fish and shellfish) is stored, it is refrigerated or frozen in a food storage unit such as a refrigerator or a refrigerated showcase. In recent years, a freshness prolonging device for foodstuff has been proposed. The freshness prolonging device stores foodstuff in a high electric field atmosphere. Such a freshness prolonging device for foodstuff is usually used together with a food storage unit to prolong a shelf life of foodstuff. For example, JP 2007-212046 A discloses a vegetable compartment of a refrigerator for storing fresh foodstuff in a high electric field atmosphere.
More specifically, JP 2007-212046 A discloses a refrigerator which can create a high electric field atmosphere in its vegetable compartment by externally applying a high voltage to a pair of electrodes mounted on, in an electrically insulated manner, a ceiling surface and a bottom surface of the vegetable compartment ([0006]). Further, JP 2007-212046 A describes that the application of a high electric field to vegetables has an effect of reducing the respiration of the vegetables ([0021]). The electrode plates are each formed by covering, with an insulating material, a metal flat plate, mesh or a resin plate having a metal film evaporated thereon ([0009] to [0011]). A high voltage to be applied to the electrode plates may be either a DC voltage or an AC voltage. However, from the viewpoint of obtaining the effect of reducing the respiration of vegetables, a voltage of 10 kV or less is preferably applied to the electrode plates to create a high electric field of 0.05 kV/cm or higher ([0021]). More specifically, it seems that a voltage of 3 to 12 kV is applied to the electrode plates ([0013]).
As described above, the electrode plates disclosed in JP 2007-212046 are each formed by covering, with an insulating material, a metal flat plate or mesh or a resin plate having a metal film evaporated thereon. In either case, the electrode plates are mainly made of a conductive metal (having a volume intrinsic resistivity on the order of 10−6 ·Ωcm). Therefore, it is necessary to apply a voltage of 3 kV or higher in order to uniformly apply a high voltage to the entire electrode plates and create a uniform high electric field atmosphere. The electrode plates disclosed in JP 2007-212046 ensures an electrically isolated state by embedding the electrode plates in a ceiling surface and a bottom surface of the vegetable compartment. It follows that JP 2007-212046 supposes non-removal electrode plates. This, however, leads to the necessity to provide the electrode plates in each food storage unit. Further, there is also a problem that the electrode plates cannot be used as a freshness prolonging device for foodstuff together with existing food storage units.